Localized surface plasmon resonances (LSPR) generated in a particle-film nanocavity enhance electric fields within a nanoscale volume. LSPR can also decay into hot carriers, highly energetic species that catalyze photocatalytic reactions in molecular analytes located in close proximity to metal surfaces. In this study, we examined the intensity of the electric field (near-field) and photocatalytic properties of plasmonic nanocavities formed by single nanoparticles (SNP) on single nanoplates (SNL). Using 4-nitrobenzenethiol (4-NBT) as a molecular reporter, we determined the near-field responses, as well as measured rates of 4-NBT dimerization into 4,4-dimercaptoazobenzene (DMAB) in the gold (Au) SNP on AuSNL nanocavity (Au-Au), as well as in AuSNP on AgSNL (Au-Ag), AgSNP on AuSNL (Ag-Au), and AgSNP on AgSNL (Ag-Ag) nanocavities using 532, 660, and 785 nm excitations. We observed the strongest near-field signals of 4-NBT at 660 nm in all examined plasmonic systems that is found to be substantially red-shifted relative to the LSPR of the corresponding nanoparticles. We also found that rates of DMAB formation were significantly greater in heterometal nanocavities (Au-Ag and Ag-Au) compared to their monometallic counterparts (Au-Au and Ag-Ag). These results point to drastic differences in plasmonic and photocatalytic properties of mono and bimetallic nanostructures.